Switching units, (e.g., circuit breakers), are used inter alia for safe disconnection in the event of a short circuit and thus protect consumers and installations. Furthermore, electrical or mechanical switching units are suitable for the operationally dependent, manual switching of consumers and for the safe isolation of an installation from the electrical grid in the event of maintenance work or changes to the installation. Electrical switching units may be operated electromagnetically.
In other words, such switching units are electrical switching devices, which are high quality in technical terms with integrated protection for motors, lines, transformers, and generators. They are used at service facilities with a relatively low switching frequency. In addition to short-circuit protection, such switching units are also suitable for overload protection.
In the event of a short circuit, an electrical switching unit disconnects an electrical installation safely. Thus, this electrical switching unit provides safety protection from overload. Any conductor through which current is flowing is heated to a greater or lesser extent. The heating is in this case dependent on the ratio of the current intensity to the conductor cross section, known as the current density. The current density should not become too great because otherwise the conductor insulation may be scorched by excessive heating and possibly a fire may be triggered.
Circuit breakers have two tripping mechanisms, which act independently of one another for overload and short-circuit protection. Both releases are connected in series. In the event of a short circuit, an electromagnetic release, which acts virtually without any delay in time, performs the function of protection. In the event of a short circuit, the electromagnetic release unlatches a switching mechanism of the circuit breaker without any delay. A switching armature isolates the switching piece before the short-circuit current may reach its maximum value.
In circuit breakers having a high switching capacity (ICU up to 100 kA at a rated current of IN=100 A), high short circuits occur, with the result that in turn large magnetic forces arise between the moving switching piece and the fixed switching piece. These are, for the one part, current-loop forces between the fixed switching pieces and the moving switching pieces and, for the other part, the much greater current constriction forces between the silver contacts. These two forces have the effect that, in the event of a short circuit, the moving switching piece is thrown suddenly counter to the resulting spring force thereof and strikes the impact domes in the lower part.
Furthermore, the abovementioned current constriction forces between the silver contacts do not act centrally in the middle of the contacts but decentrally further toward the periphery of the contacts. In addition to the opening force in the direction of the impact domes in the lower part, the moving switching piece therefore experiences a torque, which may result in rotation of the moving switching piece about the longitudinal axis. The striking of the impact domes, too, may have the effect, given an unfavorable position of the moving switching piece, that an undesired torque is again initiated.
Under extreme load it is occasionally possible for the moving switching piece to exploit its design-determined degrees of freedom, and, as a result of the initiated torques, for it either to become wedged in the contact slide or to rotate so far that regular contacting between the contacts of the moving switching piece and the fixed switching piece is no longer possible. In any case, reliable operation of the circuit breaker is no longer provided.
Known contact slides of these switching units frequently have two guide systems, namely an internal guide system and an external guide system. The external guide system is used when the switching operation, (e.g., the switch-on or switch-off operation), takes place via a switching mechanism of the switching unit. In this case, no bridge rotator occurs.
The internal guide system is used in the event of a short circuit when the switching operation is performed via a switching armature, (e.g., a plunger), of the switching unit. In other words, in the event of disconnection on account of a short circuit, the moving switching piece runs ahead of the contact slide along the internal guide system, rebounds at impact faces in what is referred to as the lower part of the switching unit and flies back along the internal guide system again. In this case, the moving switching piece flies in the opposite direction to the switching armature or the plunger of the switching unit. In this case, it is possible for the moving switching piece and the plunger to meet one another outside their center lines, and this may lead to rotation of the moving switching piece about its longitudinal axis.
If the moving switching piece remains in the rotated state, when the switching unit is next switched on, the contacts, (e.g., silver contacts of the moving switching piece and the fixed contacts of the switching unit), no longer meet one another, with the result that failure phenomena occur. In other words, a switching piece that remains in a rotated position is disadvantageous because the switching unit is then no longer usable. A non-functioning switching piece and a non-functioning switching unit are disadvantageous for the electrical consumers and the installation in which the switching unit is installed.